There is a rapidly growing demand for high-quality optical Bragg gratings with arbitrary phase and index profiles, as these gratings are key elements in many components that are used in WDM networks. Over the past few years, several methods that improve the quality and the flexibility in the grating fabrication process have been developed. A straightforward approach is to scan a UV beam over a long phase mask in a fixed relative position to the fiber [1]. Non-uniform profiles can in this case be fabricated either by post processing the illuminated region [2] or by using a phase mask that contains the appropriate structure [3]. Complex grating structures can also be synthesized by moving the fiber slightly relative to the phase mask during the scan [4].
In 1995, a novel versatile sequential technique for venting long and complex fiber gratings was demonstrated [5], [6]. The idea was to expose a large number of small partially overlapping subgratings—each containing a few hundred periods or less—in sequence; where advanced properties such as chirp, phase shifts and apodization were introduced by adjusting the phase offset and pitch of the subgratings. The flexibility of the method relies on the fact that all grating parameters are accessible in the control software and no change of hardware such as different kinds of phase masks are needed in order to fabricate gratings of arbitrary shapes.
In the setup that was used in Ref. [5] and [6], each subgrating was created by exposing the fiber with a short UV pulse while the fiber itself was translated at a constant speed. The UV pulses were triggered by the position of the fiber relative the UV beams, which was measured by a standard helium-neon laser interferometer. However, there are several drawbacks with these previously known methods. For example, the pulse energy exhibit fairly large fluctuations, which introduces amplitude noise in the grating structure. Moreover, it is necessary to use a low average pulse energy, as optical damage otherwise may be induced in the fiber. The velocity of the translation must additionally be kept low in comparison to the pulse length to maintain good visibility. For strong gratings the fiber, therefore, has to be exposed several times with low energy instead of a single time with high energy. The velocity of the translation must additionally be kept low in comparison to the pulse length to maintain good visibility. Hence the entire writing process tends to become rather time consuming and costly. A slow velocity also results in an increase of noise due to temperature variations in the fiber. The influence of the noise due to temperature variations in the fiber will increase the slower the fiber moves. Hence, it is difficult to control the pulse shape and pulse intensity appropriately in said known methods. Further, the most of the UV radiation is not effectively used in the grating formation process, rendering the process even more inefficient and costly.